Technical Field
[0001] The present invention relates to a particulate combustion catalyst, to a particulate
filter, and to an exhaust gas cleanup system. More particularly, the present invention
relates to a particulate combustion catalyst which realizes removal (through oxidation)
of particulate matter discharged from a diesel internal combustion engine; to a particulate
filter coated with the particulate combustion catalyst; and to an exhaust gas cleanup
system including the particulate filter coated with the particulate combustion catalyst.
Background Art
[0002] Exhaust gas discharged from diesel engines contains nitrogen oxide (NO
x) and particulate matter, and release of such substances into the atmosphere without
any treatment is a main cause of air pollution. Therefore, demand has arisen for strict
regulations for such substances. Examples of effective means for removing particulate
matter include a diesel exhaust gas trapping system employing a flow-through oxidation
catalyst for combustion of soluble organic fractions (SOFs) or a diesel particulate
filter (DPF) for trapping soot. For regeneration of the DPF, particulate matter trapped
therein must be continuously removed through oxidation.
[0003] Hitherto proposed continuous regeneration systems include a system employing a catalyst
including a carrier made of an inorganic oxide (e.g., zirconium oxide, vanadium oxide,
or cerium oxide), and an expensive noble metal (e.g., Pt) supported on the carrier
(see, for example, Patent Document 1, 2, or 3) or a system employing a catalyst including
a carrier made of cerium-zirconium mixed oxide and ruthenium supported on the carrier
(see patent document 4, 5, 6 or non-patent document 1); and a continuous regeneration
method employing NO
2 (see, for example, Patent Document 7). This continuous regeneration method requires
provision, upstream of a DPF, of an oxidation catalyst (e.g., Pt) for oxidizing NO
into NO
2, and thus involves high cost. In addition, reaction involving NO
2 is affected by the ratio between NO
x and C, and many restrictions are imposed on the employment of this method.
Disclosure of the Invention
Problems to be Solved by the Invention
[0005] An object of the present invention is to provide a particulate combustion catalyst
which realizes removal of soot through oxidation at low temperature without employment
of an expensive noble metal, which enables oxidation reaction to proceed with the
aid of only oxygen and thus realizes removal of soot through oxidation at low temperature
regardless of the NO
x concentration of exhaust gas, and which, through addition of a small amount of a
noble metal, can enhance effective removal of SOFs through oxidation while ensuring
soot combustion. Another object of the present invention is to provide a particulate
filter coated with the particulate combustion catalyst. Yet another object of the
present invention is to provide an exhaust gas cleanup system comprising the particulate
filter coated with the particulate combustion catalyst.
Means for Solving the Problems
[0006] In order to achieve the aforementioned objects, the present inventors have conducted
extensive studies, and as a result have found that the objects can be achieved by
employing, as a carrier of a particulate combustion catalyst, a cerium-zirconium double
oxide having a specific composition, or a cerium-zirconium-based double oxide having
a specific composition; employing, as a catalyst component, a first catalyst component
consisting of Ag, or an oxide thereof ; and additionally employing a second catalyst
component containing at least one metal selected from among Pt, Pd, and Rh, or an
oxide of any of these metals. The present invention has been accomplished on the basis
of this finding.
[0007] Accordingly, the present invention provides a particulate combustion catalyst characterized
by comprising a carrier containing a cerium-zirconium double oxide having a cerium
oxide content of 5 to 50 mass%; and a first catalyst component supported on the carrier,
and consisting of Ag, or an oxide thereof, with second catalyst component containing
at least one metal selected from among Pt, and Pd, and Rh, or an oxide of any of these
metals.
[0008] The present invention also provides a particulate combustion catalyst characterized
by comprising a carrier containing a cerium-zirconium-based double oxide containing
an oxide of at least one metal selected from among Nd, La, Y, Pr, Ba, Ca, Mg, Sn,
and Sr in an amount of 1 to 35 mass%, and having a cerium oxide content of 5 to 50
mass%; and a first catalyst component supported on the carrier, and consisting of
Ag, or an oxide thereof, with second catalyst component containing at least one metal
selected from among Pt and Pd and Rh, or an oxide of any of these metals.
[0009] In any of the aforementioned particulate combustion catalysts of the present invention,
a second catalyst component containing at least one metal selected from among Pt,
Pd, and Rh, or an oxide of any of these metals is additionally supported on the carrier
in an amount as reduced to metal of 0.01 to 2 mass% on the basis of the mass of the
carrier.
[0010] The present invention also provides a particulate filter characterized by being coated
with any of the aforementioned particulate combustion catalysts. The present invention
also provides an exhaust gas cleanup system characterized by comprising a particulate
filter coated with any of the aforementioned particulate combustion catalysts.
Effects of the Invention
[0011] Employment of the particulate combustion catalyst of the present invention realizes
removal of soot through oxidation at low temperature without employment of an expensive
noble metal. When the combustion catalyst is employed, since oxidation reaction proceeds
with the aid of only oxygen, soot can be removed through oxidation at low temperature
regardless of the NO
x concentration of exhaust gas. Even when a catalyst system employing the catalyst
is exposed to a high-temperature atmosphere for a long period of time, degradation
of the system can be suppressed. Addition of a small amount of a noble metal can enhance
effective removal of SOFs through oxidation while ensuring soot combustion.
Best Modes for Carrying Out the Invention
[0012] In the present invention, a cerium-zirconium double oxide having a specific composition
is employed as a carrier of the particulate combustion catalyst. The cerium oxide
content of the double oxide must be 5 to 50 mass%. When the cerium oxide content exceeds
50 mass%, the specific surface area of the carrier is considerably reduced at a high
temperature (e.g., 700°C or higher), which eventually results in thermal degradation
of the catalyst. In addition, when the cerium oxide content exceeds 50 mass%, an active
species fails to sufficiently exert its performance. In contrast, when the cerium
oxide content is less than 5 mass%, the carrier exhibits poor heat resistance, which
may eventually result in thermal degradation of the catalyst.
[0013] In the present invention, preferably, the carrier is made of a cerium-zirconium-based
double oxide containing an oxide of at least one metal selected from among Nd, La,
Y, Pr, Ba, Ca, Mg, Sn, and Sr. When the carrier is made of a cerium-zirconium-based
double oxide containing an oxide of such a metal, the carrier exhibits improved thermal
stability, and oxidation property at low temperature is improved. In order to attain
such effects, the amount of an oxide of at least one metal selected from among Nd,
La, Y, Pr, Ba, Ca, Mg, Sn, and Sr must be 1 mass% or more. However, when the amount
of such a metal oxide exceeds 35 mass%, accordingly, the relative amounts of cerium
oxide and zirconium oxide are reduced, and characteristics of a carrier made of a
cerium-zirconium double oxide tend to be deteriorated. Therefore, in the cerium-zirconium-based
double oxide contained in the carrier employed, preferably, the amount of an oxide
of at least one metal selected from among Nd, La, Y, Pr, Ba, Ca, Mg, Sn, and Sr is
1 to 35 mass% (i.e., when two or more metal oxides are employed, the total amount
of the oxides is 1 to 35.mass%), and the cerium oxide content is 5 to 50 mass%.
[0014] In the present invention, Ag or an oxide thereof must be supported, as a first catalyst
component, on the carrier. A conventionally known technique (e.g., the impregnation
method or the sol-gel method) may be employed for providing the first catalyst component
on the carrier. Ag, which is employed in the present invention, is less expensive
than, for example, Pt or Pd. In addition, when an Ag component is employed in combination
with a specific carrier used in the present invention, further excellent effects are
obtained, as compared with the case where a Pt or Pd component is employed. In the
present invention, preferably, the amount (as reduced to metal) of the first catalyst
component; i.e., Ag, or an oxide thereof. , is 0.5 to 30 mass% on the basis of the
mass of the carrier (i.e., 0.5 to 30 parts by mass on the basis of 100 parts by mass
of the carrier). When the amount of the first catalyst component is less than 0.5
mass%, the catalyst component fails to sufficiently exhibit its catalytic effects,
whereas when the amount of the first catalyst component exceeds 30 mass%, the catalyst
component-carrier combination employed in the present invention (i.e., the combination
of a specific catalyst component and a specific carrier) fails to sufficiently exhibit
a synergistic effect. Meanwhile, when the amount of the catalyst component is large,
sintering of metal is likely to occur, and the catalyst component is not expected
to exhibit its catalytic effects.
[0015] In the present invention, a second catalyst component; i.e., at least one metal selected
from among Pt, Pd, and Rh, or an oxide of any of these metals, is additionally supported
on the carrier in an amount (as reduced to metal) of 0.01 to 2 mass% on the basis
of the mass of the carrier, so as to enhance removal of SOFs contained in particulate
matter through oxidation. When the amount of the second catalyst component exceeds
2 mass%, the resultant catalyst becomes expensive, since Pt, Pd, or Rh is an expensive
metal. In addition, Ag fails to sufficiently exhibit its catalytic effects. In contrast,
when the amount of the second catalyst component is less than 0.01 mass%, the catalyst
component fails to sufficiently exhibit its catalytic effects.
[0016] In consideration that the particulate filter of the present invention is produced
by causing the particulate combustion catalyst of the present invention to be held
on a base, preferably, the surface of the carrier is provided with a binder component
such as SiO
2, TiO
2, ZrO
2, or Al
2O
3. When such a binder component is provided on the surface of the carrier, adhesion
between the base and the carrier is enhanced, and the catalyst exhibits improved durability
and heat resistance.
[0017] The particulate filter of the present invention may assume any known form of particulate
filter, but preferably has a three-dimensional structure. Specific examples of filters
having a three-dimensional structure include a wall-through filter, a flow-through
honeycomb filter, a wire mesh filter, a ceramic fiber filter, a metallic porous filter,
a particle-charged filter, and a foam filter. Examples of the material of the base
include ceramic materials such as cordierite and SiC; Fe-Cr-Al alloys; and stainless
steel alloys.
The exhaust gas cleanup system of the present invention, into which the aforementioned
particulate filter of the present invention has been incorporated, will be readily
appreciated by those skilled in the art.
[0018] Next will be described a method for producing the particulate filter of the present
invention.
A cerium-zirconium double oxide serving as a carrier is mixed with a binder component
(e.g., SiO
2 or alumina sol) and water, and the resultant mixture is finely milled by means of
a milling apparatus (e.g., a ball mill). A base filter (e.g., a wire mesh filter)
is coated with the thus-obtained slurry. In general, the slurry-coated filter is fired
at a temperature of about 500°C to about 700°C. The thus-formed wash-coating layer
is impregnated with a catalyst component (e.g., a nitrate of ruthenium or silver),
followed by drying and firing. The total catalyst coating amount is preferably 10
to 100 g/L (for a wall-flow DPF) or about 50 to about 150 g/L (for a wire mesh DPF).
When the total catalyst coating amount is excessively small, sufficient performance
fails to be attained, whereas when the total catalyst coating amount is excessively
large, back pressure to exhaust gas increases. Examples
[0019] The present invention will next be described in detail with reference to Examples
and Comparative Examples. In each of the Examples and Comparative Examples, a parenthesized
numerical value following each of the oxides constituting a double oxide represents
the amount (mass%) of the constitutive oxide.
Comparative Example 1
[0020] Water (30 g) was added to a nitric acid solution (concentration: 4 mass%) (1.25 g)
of ruthenium nitrate, and powder (5.0 g) of a double oxide of CeO
2(21) + ZrO
2(72) + La
2O
3(2) + Nd
2O
3 (5) was added thereto, followed by stirring for 30 minutes. The thus-obtained slurry
was dried at 120°C for three hours, and then finally fired in air at 500°C for one
hour. The resultant particulate combustion catalyst was found to have an Ru content
of 1 mass% on the basis of the mass of the carrier.
Example 1 (not according to the invention)
[0021] Water (30 g) was added to silver nitrate (0.080 g), followed by stirring, to thereby
prepare an aqueous silver nitrate solution. Powder (5.0 g) of a double oxide of CeO
2(21) + ZrO
2(72) + La
2O
3(2) + Nd
2O
3(5) was added to the aqueous solution, followed by stirring for 30 minutes. The thus-obtained
slurry was dried at 120°C for three hours, and then finally fired in air at 500°C
for one hour. The resultant particulate combustion catalyst was found to have an Ag
content of 1 mass% on the basis of the mass of the carrier.
Example 2 (not according to the invention)
[0022] Water (30 g) was added to silver nitrate (0.417 g), followed by stirring, to thereby
prepare an aqueous silver nitrate solution. Powder (5.0 g) of a double oxide of CeO
2(30) + ZrO
2(70) was added to the aqueous solution, followed by stirring for 30 minutes. The thus-obtained
slurry was dried at 120°C for three hours, and then finally fired in air at 500°C
for one hour. The resultant particulate combustion catalyst was found to have an Ag
content of 5 mass% on the basis of the mass of the carrier.
Example 3 (not according to the invention)
[0023] The procedure of Example 2 was repeated, except that the double oxide of CeO
2(30) + ZrO
2(70) was replaced by a double oxide of CeO
2(30) + ZrO
2(62) + SnO
2(8), to thereby prepare a particulate combustion catalyst. The particulate combustion
catalyst was found to have an Ag content of 5 mass% on the basis of the mass of the
carrier.
Example 4 (not according to the invention)
[0024] The procedure of Example 2 was repeated, except that the double oxide of CeO
2(30) + ZrO
2(70) was replaced by a double oxide of CeO
2(30) + ZrO
2(65) + BaO(5), to thereby prepare a particulate combustion catalyst. The particulate
combustion catalyst was found to have an Ag content of 5 mass% on the basis of the
mass of the carrier.
Example 5 (not according to the invention)
[0025] The procedure of Example 2 was repeated, except that the double oxide of CeO
2(30) + ZrO
2(70) was replaced by a double oxide of CeO
2(33) + ZrO
2(65) + SrO(2), to thereby prepare a particulate combustion catalyst. The particulate
combustion catalyst was found to have an Ag content of 5 mass% on the basis of the
mass of the carrier.
Example 6 (not according to the invention)
[0026] The procedure of Example 2 was repeated, except that the double oxide of CeO
2(30) + ZrO
2(70) was replaced by a double oxide of CeO
2(47) + ZrO
2(47) + La
2O
3(2) + Nd
2O
3(4), to thereby prepare a particulate combustion catalyst. The particulate combustion
catalyst was found to have an Ag content of 5 mass% on the basis of the mass of the
carrier.
Comparative Example 2
[0027] Water (30 g) was added to a nitric acid solution (concentration: 4 mass%) (6.63 g)
of ruthenium nitrate, and powder (5.0 g) of a double oxide of CeO
2(47) + ZrO
2(47) + La
2O
3(2) + Nd
2O
3(4) was added thereto, followed by stirring for 30 minutes. The thus-obtained slurry
was dried at 120°C for three hours, and then finally fired in air at 500°C for one
hour. The resultant particulate combustion catalyst was found to have an Ru content
of 5 mass% on the basis of the mass of the carrier.
Example 7 (not according to the invention)
[0028] The procedure of Example 2 was repeated, except that the double oxide of CeO
2(30) + ZrO
2(70) was replaced by a double oxide of CeO
2(20) + ZrO
2(72) + Y
2O
3(2) + Pr
6O
11(6) , to thereby prepare a particulate combustion catalyst. The particulate combustion
catalyst was found to have an Ag content of 5 mass% on the basis of the mass of the
carrier.
Example 8 (not according to the invention)
[0029] The procedure of Example 2 was repeated, except that the double oxide of CeO
2(30) + ZrO
2(70) was replaced by a double oxide of CeO
2(8) + ZrO
2(62) + La
2O
3(2) + Nd
2O
3(28), to thereby prepare a particulate combustion catalyst. The particulate combustion
catalyst was found to have an Ag content of 5 mass% on the basis of the mass of the
carrier.
Example 9 (not according to the invention)
[0030] The procedure of Example 2 was repeated, except that the double oxide of CeO
2(30) + ZrO
2(70) was replaced by a double oxide of CeO
2(21) + ZrO
2(72) + La
2O
3(2) + Fe
2O
3(5) , to thereby prepare a particulate combustion catalyst. The particulate combustion
catalyst was found to have an Ag content of 5 mass% on the basis of the mass of the
carrier.
Comparative Example 3
[0031] The procedure of Comparative Example 1 was repeated, except that the nitric acid
solution of dinitrodiammine Pt was replaced by silver nitrate, to thereby prepare
a particulate combustion catalyst having an Ag content of 1 mass% on the basis of
the mass of the carrier.
Comparative Example 4
[0032] The procedure of Comparative Example 1 was repeated, except that the nitric acid
solution of ruthenium nitrate was replaced by silver nitrate, and the double oxide
was replaced by a double oxide of CeO
2(58) + ZrO
2(42), to thereby prepare a particulate combustion catalyst having an Ag content of
5 mass% on the basis of the mass of the carrier.
Comparative Example 5
[0033] The procedure of Comparative Example 4 was repeated, except that the double oxide
of CeO
2(58) + ZrO
2(42) was replaced by a double oxide of CeO
2(75) + ZrO
2(18) + La
2O
3(2) + Nd
2O
3(5), to thereby prepare a particulate combustion catalyst having an Ag content of
5 mass% on the basis of the mass of the carrier.
Comparative Example 6
[0034] The procedure of Comparative Example 1 was repeated, except that the nitric acid
solution of ruthenium nitrate was replaced by a nitric acid solution of dinitrodiammine
Pt, to thereby prepare a particulate combustion catalyst having a Pt content of 1
mass% on the basis of the mass of the carrier.
Evaluation of powdery catalyst by use of simulated exhaust gas
[0035] Combustion initiation temperature corresponding to each of the powdery particulate
combustion catalysts prepared in Examples 1 to 9 and Comparative Examples 1 to 6 was
measured through the following method.
[0036] Each of the powdery particulate combustion catalysts prepared in Examples 1 to 9
and Comparative Examples 1 to 6 (50 mg) and carbon (Printex-V (toner carbon), product
of Degussa) (5 mg) were mixed together by means of an agate mortar for 15 seconds,
and the resultant mixture was fixed with quartz wool at a center portion of a quartz
reaction tube. While a circulation gas having the below-described composition was
caused to flow through the quartz reaction tube at the below-described flow rate,
the temperature of the reaction tube was elevated at the below-described temperature
elevation rate by means of an electric furnace, and CO and CO
2 concentrations were measured at the outlet of the reaction tube by means of an infrared
analyzer. The temperature at the inlet of the catalyst-containing reaction tube was
measured when CO
2 concentration reached 100 ppm (i.e., electric furnace control temperature). This
temperature was employed as combustion initiation temperature.
Gas composition: O
2: 10%, H
2O: 10%, N
2: balance
Flow rate: 4,000 cc/min
Temperature elevation rate: 10 degrees (°C)/min
[0037] Table 1 shows the thus-measured combustion initiation temperatures corresponding
to the respective powdery particulate combustion catalysts prepared in Examples 1
to 9 and Comparative Examples 1 to 6, catalytically active species of the catalysts;
amounts of the catalytically active species (mass% on the basis of the entire carrier
mass); and carrier compositions.
[0038] [Table 1]
Table 1
|
Catalytically active species |
Supported catalyst amount mass% |
Carrier composition (mass%) |
Combustion initiation temp. (°C) |
Comp. Ex. 1 |
Ru |
1 |
CeO2(21)+ZrO2(72)+La2O3(2)+Nd2O3(5) |
281 |
Ex. 1 |
Ag |
1 |
CeO2(21)+ZrO2(72)+La2O3(2)+Nd2O3(5) |
261 |
Ex. 2 |
Ag |
5 |
CeO2(30)+ZrO2(70) |
237 |
Ex. 3 |
Ag |
5 |
CeO2(30)+ZrO2(62)+SnO2(8) |
246 |
Ex. 4 |
Ag |
5 |
CeO2(30)+ZrO2(65)+BaO(5) |
257 |
Ex. 5 |
Ag |
5 |
CeO2(33)+ZrO2(65)+SrO(2) |
260 |
Ex. 6 |
Ag |
5 |
CeO2(47)+ZrO2(47)+La2O3(2)+Nd2O3(4) |
232 |
Comp. Ex. 2 |
Ru |
5 |
CeO2(47)+ZrO2(47)+La2O3(2)+Nd2O3(4) |
263 |
Ex. 7 |
Ag |
5 |
CeO2(20)+ZrO2(72)+Y2O3(2)+Pr6O11(6) |
231 |
Ex. 8 |
Ag |
5 |
CeO2(8)+ZrO2(62)+La2O3(2)+Nd2O3(28) |
256 |
Ex. 9 |
Aq |
5 |
CeO2(21)+ZrO2(72)+La2O3(2)+Fe2O3(5) |
243 |
Comp. Ex. 3 |
Ag |
1 |
ZrO2 |
403 |
Comp. Ex. 4 |
Ag |
5 |
CeO2(58)+ZrO2(42) |
356 |
Comp. Ex. 5 |
Ag |
5 |
CeO2(75)+ZrO2(18)+La2O3(2)+Nd2O3(5) |
340 |
Comp. Ex. 6 |
Pt |
1 |
CeO2(21)+ZrO2(72)+La2O3(2)+Nd2O3(5) |
351 |
Example 10 (not according to the invention)
[0039] Water (30 g) was added to a double oxide (20 g) of CeO
2(47) + ZrO
2(47) + La
2O
3(2) + Nd
2O
3(4), and SiO
2 sol (i.e., a binder component) (5 g as reduced to SiO
2) was added thereto, followed by mixing for two hours, to thereby prepare a wash-coat
slurry. A wire mesh filter (20 mm in diameter x 20 mm in length, wire diameter: 0.25
mm) was coated with the slurry, followed by drying at 120°C for three hours, and then
firing in air at 500°C for one hour. Thus, a double oxide carrier layer containing
the binder component was formed on the filter so that the amount of the carrier was
100 g on the basis of 1 L of filter volume. The double-oxide-coated filter was impregnated
with an aqueous silver nitrate solution having a predetermined concentration. The
resultant product was dried at 120°C for three hours, and then finally fired in air
at 500°C for one hour. The thus-obtained particulate filter was found to have an Ag
content of 5 g on the basis of 1 L of the filter, or 5 mass% on the basis of the mass
of the carrier.
Evaluation of catalyst-coated particulate filter by use of simulated exhaust gas
[0040] Combustion initiation temperature corresponding to the catalyst-coated particulate
filter obtained in Example 10 was measured through the following method.
[0041] A predetermined amount of a dispersion prepared by dispersing carbon (Printex-V
(toner carbon), product of Degussa) (20 mg) in ethyl alcohol was added dropwise to
the catalyst-coated particulate filter obtained in Example 10 (20 mm in diameter x
20 mm in length) from above the filter, followed by drying at 100°C for 10 minutes.
Thus, carbon (20 mg) was deposited on one catalyst-coated particulate filter. The
carbon-deposited filter was fixed at a center portion of a quartz reaction tube. While
a circulation gas having the below-described composition was caused to flow through
the quartz reaction tube at the below-described flow rate, the temperature of the
reaction tube was elevated at the below-described temperature elevation rate by means
of an electric furnace, and CO and CO
2 concentrations were measured at the outlet of the reaction tube by means of an infrared
analyzer. The temperature at the inlet of the catalyst-containing reaction tube was
measured when CO
2 concentration reached 400 ppm (i.e., electric furnace control temperature). This
temperature was employed as combustion initiation temperature. The combustion initiation
temperature was found to be 396°C.
Gas composition: O
2: 10%, H
2O: 10%, N
2: balance
Flow rate: 4,000 cc/min
Temperature elevation rate: 10 degrees (°C)/min
Example 11 (not according to the invention)
[0042] Water (30 g) was added to a double oxide (20 g) of CeO
2(30) + ZrO
2(70), and acetic acid-ZrO
2 sol serving as a binder component (5 g as reduced to ZrO
2) was added thereto, followed by mixing for two hours, to thereby prepare a wash-coat
slurry. A cordierite wall-flow particulate filter (DPF) (25.4 mm in diameter x 60
mm in length) was coated with the oxide by use of the slurry. The resultant product
was dried at 120°C for three hours, and then finally fired in air at 500°C for one
hour. Thus, a double oxide carrier layer containing the binder component was formed
on the filter so that the amount of the carrier was 50 g on the basis of 1 L of filter
volume. The double-oxide-coated filter was impregnated with an aqueous silver nitrate
solution having a predetermined concentration. The resultant product was dried at
120°C for three hours, and then finally fired in air at 500°C for one hour. The finally
formed catalyst was found to have an Ag content of 5 mass% on the basis of the mass
of the carrier.
Example 12
[0043] The procedure of Example 11 was repeated, except that a mixture of an aqueous silver
nitrate solution having a predetermined concentration and a nitric acid solution of
dinitrodiammine Pt having a predetermined concentration was employed, to thereby coat
a double-oxide-coated cordierite DPF with a catalyst component. The finally formed
catalyst was found to have an Ag content of 5 mass% and a Pt content of 0.1 mass%,
on the basis of the mass of the carrier.
Example 13
[0044] The procedure of Example 11 was repeated, except that a mixture of an aqueous silver
nitrate solution having a predetermined concentration and an aqueous palladium nitrate
solution having a predetermined concentration was employed, to thereby coat a double-oxide-coated
cordierite DPF with a catalyst component. The finally formed catalyst was found to
have an Ag content of 5 mass% and a Pd content of 1 mass%, on the basis of the mass
of the carrier.
Example 14
[0045] The procedure of Example 13 was repeated, except that CeO
2(20) + ZrO
2(70) + CaO(10) was employed as a double oxide, to thereby coat a double-oxide-coated
cordierite DPF with a catalyst component. The finally formed catalyst was found to
have an Ag content of 5 mass% and a Pd content of 0.1 mass%, on the basis of the mass
of the carrier.
Example 15
[0046] The procedure of Example 13 was repeated, except that CeO
2(20) + ZrO
2(70) + MgO(10) was employed as a double oxide, to thereby coat a double-oxide-coated
cordierite DPF with a catalyst component. The finally formed catalyst was found to
have an Ag content of 5 mass% and a Pd content of 0.1 mass%, on the basis of the mass
of the carrier.
Evaluation of catalyst-coated DPF in terms of soot combustion by use of simulated
exhaust gas
[0047] Soot combustion initiation temperature corresponding to each of the catalyst-coated
DPFs obtained in Examples 11 to 15 was measured through the following method.
[0048] A predetermined amount of a dispersion prepared by dispersing carbon (Printex-V (toner
carbon), product of Degussa) in ethyl alcohol was added dropwise to each of the catalyst-coated
DPFs (25.4 mm in diameter x 60 mm in length) obtained in Examples 11 to 15 from above
the DPF, followed by drying at 100°C for 30 minutes. Thus, carbon (150 mg) was deposited
on one catalyst-coated DPF. The carbon-deposited DPF was fixed at a center portion
of a simulated exhaust gas reaction tube (quartz reaction tube). While a circulation
gas having the below-described composition was caused to flow through the quartz reaction
tube at the below-described flow rate, the temperature of the reaction tube was elevated
to 600°C at the below-described temperature elevation rate by means of an electric
furnace, and CO and CO
2 concentrations were measured at the outlet of the reaction tube by means of an infrared
analyzer. The temperature at the inlet of the catalyst-containing reaction tube was
measured when CO
2 concentration reached 30 ppm (i.e., electric furnace control temperature). This temperature
was employed as combustion initiation temperature.
Gas composition: NO: 200 ppm, O
2: 10%, H
2O: 10%, N
2: balance
Flow rate: 25 L/min
Temperature elevation rate: 20 degrees (°C)/min
[0049] Evaluation of catalyst-coated DPF in terms of combustion cleaning of light oil by
use of simulated exhaust gas
Light oil cleanup temperature corresponding to each of the catalyst-coated DPFs obtained
in Examples 11 to 15 was measured through the below-described method, for evaluation
of light oil cleanup property (temperature at which percent cleanup reaches 50%).
[0050] Each of the catalyst-coated particulate filters obtained in Examples 11 to 15 was
fixed at a center portion of a simulated exhaust gas reaction tube (quartz reaction
tube). While a circulation gas having the below-described composition was caused to
flow through the quartz reaction tube at the below-described flow rate, the temperature
of the reaction tube was elevated to 450°C at the below-described temperature elevation
rate by means of an electric furnace, and THC (total hydrocarbon content) was determined
by means of an analyzer. In the determination, hydrocarbon concentration was measured
at the outlet of the reaction tube by means of a hydrogen flame detector. The thus-obtained
temperature values at which percent THC cleanup reached 50% (i.e., T50) were compared
with one another.
Gas composition: NO: 200 ppm, O
2: 10%, CO: 300 ppm, hydrogen: 100 ppm, H
2O: 10%, N
2: balance
Light oil: mass ratio, 2.5 times the mass of NO
x
Flow rate: 25 L/min
Temperature elevation rate: 20 degrees (°C)/min
[0051] Table 2 shows the thus-measured combustion initiation temperatures corresponding
to the respective catalyst-coated particulate filters obtained in Examples 11 to 15;
hydrocarbon 50% cleanup temperatures corresponding to the filters; catalytically active
species of the catalysts; amounts of the catalytically active species (mass% on the
basis of the entire carrier mass); and carrier compositions.
[0052] [Table 2]
Table 2
|
Catalytically active species |
Supported catalyst amount mass% |
Carrier composition (mass%) |
Combustion initiation temp. (°C) |
HC-T50 temp. (°C) |
Ex. 11 |
Ag |
5 |
CeO2(30)+ZrO2(70) |
387 |
343 |
Ex 12 |
Ag/Pt |
5/0.1 |
CeO2(30)+ZrO2(70) |
383 |
302 |
Ex. 13 |
Ag/Pd |
5/1 |
CeO2(30)+ZrO2(70) |
388 |
248 |
Ex. 14 |
Ag/Pd |
5/0.1 |
CeO2(20)+ZrO2(70)+CaO(10) |
383 |
318 |
Ex. 15 |
Ag/Pd |
5/0.1 |
CeO2(20)+ZrO2(70)+MgO(10) |
385 |
322 |